The present invention relates to connections between and methods of connecting field windings and exciter or slip rings in dynamoelectric machines and particularly relates to a sectional terminal stud for electrically interconnecting a bore copper and field windings.
The rotors of dynamoelectric machines typically comprise relatively large diameter cylindrical bodies containing field windings for producing magnetic flux which in turn produces stator current and voltage. These field windings are normally carried in a series of longitudinal slots along the outer circumference and extend the length of the rotor body. Rotation of the body particularly at speeds of 3600 rpm, for example, exerts high centrifugal forces on the windings. These windings are conventionally retained in the rotor slots through the use of dovetail shaped wedges which also extend along the length of the rotor body. The manner in which the windings and rotor slots are shaped, insulated and cooled present formidable design problems, particularly for units designed for long term operation under variable load and environmental conditions. Because the windings extend axially beyond the rotor body and wedge ends and are subjected to the same rotational forces which tend to thrust the winding end turns in a radially outward direction, specially designed structure must be included to prevent such radial movement, as well as for making electrical connections between the exciter or slip rings and the windings.
As to the problem of preventing radial movement of the end turns, it is conventional to enclose the winding end turns within retaining rings attached to the rotor body ends by shrink fitting such rings around circumferential lips at the ends of the rotor body. Other means, such as locking keys and the like, are additionally included to maintain the retaining rings securely on the rotor so as to counteract the effects of thermal expansion on the retaining rings.
As to the manner in which electrical connections may be made between the field windings and bore connectors, also known as the xe2x80x9cbore copperxe2x80x9d (insulated conductors embedded in small diameter shafts that extend from opposite ends of the rotor body for electrical connection with the exciter/rectifier assembly), such field winding connections as found in the prior art conventionally utilize brazed leaves of copper bent to various configurations. These configurations have exhibited premature failures due to cyclic mechanical and electrical duty requirements, which require the connectors to have particular characteristics.
More particularly, in a current long-standing design affording electrical connection between a field winding and a bore conductor (see, for example, U.S. Pat. No. 5,358,432), a main terminal is inserted into a radial bore of the shaft. The main terminal has tapered threads at its radial inner end for engaging female tapered threads in the bore connector. Tolerances of the taper angle, thread pitch, the contacts along minimum major and minor threaded diameters and the need to torque the main terminal to a predetermined value render the installation of the main terminal to bore connector connection difficult as well as affording less than optimal conditions for good electrical connection. The opposite end of the main terminal includes a plurality of flexible leaves which are electrically connected to the field winding. Because of the pipe thread type securement between the main terminal and bore connector, the thin terminal leaves are necessarily formed and brazed together in the field. In that process, care must be taken not to melt the necessary thin copper leaves or to allow the brazing alloy to migrate into the flexible part of the terminal. Field brazing of the leaves to one another and to the field winding is time consuming and laborious. Should the leaves melt in the course of brazing or should braze alloy migrate to the flexible part of the terminal, the high rotational and thermal forces of the rotor will cause the flexible connection to prematurely fail causing unscheduled outages and generator down time.
In a prior application of assignee hereof, an improved terminal stud connector between the bore copper and field winding is disclosed. In that invention, the radially open threaded aperture of the bore copper is machined to remove the threads and receive an annular spring-like electrical connector for receiving and making electrical contact with the radial inner end of the terminal stud. While that arrangement is satisfactory for new generators, older generators undergoing retrofit in the field require the complete removal and remanufacture of the bore copper embedded in the center of the rotor in order to utilize the improved terminal connector of that application. Because it is not economical to remove and remanufacture the bore connector in order to retrofit such terminal stud, there is a need to improve the connection between the bore copper and field windings for original equipment as well as in a manner rendering the repair and refurbishment of generators in the field and in this generator location more economical, practical and more speedily effected.
In accordance with a preferred embodiment of the present invention, there is provided a method and apparatus for repairing generators in the field without the need for remanufacturing and/or removal of the bore connector from the center of the rotor. To accomplish the foregoing, a preferred form of the present invention provides a sectional terminal stud including a radially inner terminal stud adapter section and an outer terminal stud section. The adapter section has on its radially inner end a radially inner, male threaded portion to match the female threaded portion of the existing bore copper whereby the terminal stud adapter can be readily threaded into the bore copper in the field. The opposite end of the adapter section includes a recess for receiving an annular spring-like electrical connector. The outer section includes a radial inner end receivable in the recess within the annular spring and which spring affords good electrical connection between the outer and adapter sections of the terminal stud. The opposite end of the outer section includes the leaf copper for connection to the field windings.
It will be appreciated that a principal difficulty with replacing a terminal stud where the terminal stud is screwthreaded into the bore copper is the necessity to orient the terminal stud about its axis so that the leaf copper on the outer end of the stud is oriented relative to the axial centerline of the machine. In other words, by using a threaded connection between the terminal stud and the bore copper, the resulting rotational orientation of the terminal stud is problematic. To cure that problem, the leaf copper was typically applied to the terminal stud but left unformed and unattached to the windings until it was assembled into the bore copper and a specific torque was applied to secure the terminal stud and bore copper to one another. The leaf copper was then formed to fit the coil, and the stud was over or under-torqued to achieve alignment. However, it has been found that such over-torque or under-torque leads to premature failures and, because of this type of connection, the leaf copper was subjected to low-cycle fatigue mechanism. Consequently, by forming a terminal stud of two sections with the radially inner adapter section screwthreaded into the bore copper and without requiring rotational orientation of the outer section relative to the adapter section, the leaf copper can be formed on the outer section of the terminal stud prior to installation. In this manner, the outer section and the leaf copper secured thereto can simply be rotated for alignment with the machine and the field windings.
In a preferred embodiment according to the present invention, there is provided in a dynamoelectric machine having a rotor, a radial bore in the rotor, a bore connector extending generally in an axial direction along a portion of the rotor and having a generally radially outwardly opening threaded aperture, a terminal stud in the radial bore for electrically connecting the bore connector and field windings of the machine and including an adapter section and an outer section, the adapter section having a threaded end portion for threaded engagement with the bore connector in the threaded aperture thereof, an opposite end of the adapter section having a recess, the outer section having an electrical contact portion at one end thereof for electrical connection with the adapter portion, a multi-contact electrical connector in the recess for electrically coupling the sections to one another, the sections being axially aligned with one another with the electrical contact portion of the outer section received in the recess without rotational orientation of the outer section, the outer section having a leaf copper at an opposite end thereof for engaging the field windings of the machine whereby the outer section is assembled to the adapter section without regard to the rotational orientation of the adapter section relative to the bore connector caused by the threaded connection between the adapter section and bore connector.
In a further preferred embodiment according to the present invention, there is provided in a dynamoelectric machine having a rotor and a radial bore in the rotor, a bore connector extending generally in an axial direction along a portion of the rotor and having a generally radially outwardly opening aperture and a terminal stud including an adapter section and an outer section, a method of connecting the bore connector and a field winding, comprising the steps of (a) screwthreading a threaded end of the adapter section of the terminal stud into the radially outwardly opening aperture of the bore connector, (b) inserting one end portion of the outer section of the terminal stud into a recess formed on the opposite end of the adapter section from the threaded end to effect an electrical contact between the adapter section and the outer section and (c) securing leaf copper on an opposite end of the outer section to the field winding.